a lowpass converter would get the job done nicely - sounds like you want a sq (sound quality) sub, so your best bets are 1 infinity perfect 10.i, audiobahn aw1000, or idq/boston actoustic/various others depending on how much you want to spend. A perfect 10.i will run about $140-150, and the aw1000, in my opinion the best deal for subs, is less than $100. I would get a 2 channel amp that does about 250-300 rms at 4 ohms bridged. Hifonics, Autotek, Mtx, all are good low-priced options. Infinity, blaupunkt, memphis, others are better high priced amps. Hifonics is my personal favorite. If you're adding a sub to your factory system, you may find that the bass is overpowering your other speakers - my recommendation to you is to keep your head unit, get a four channel amp, then a good set of components or coax's with good midbass and excellent highs (boston rally, infinity kappa, infinity perfect), run them off two channels on the amp, take out your rear speakers (you really dont need them since you're putting in a sub), and put the sub on the remaining two channels, bridged. This way your highs/mids wont be overpowered by your bass, and your sound system will be dramatically better. Also, by taking out the rear speakers, you are allowing much more bass to enter the cabin from your sub in the trunk.